Boron removal and reclamation by magnetic magnetite (Fe3O4) nanoparticle: An adsorption and isotopic separation study

Separation and Purification Technology 231 (2020) 115930

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Boron removal and reclamation by magnetic magnetite (Fe3O4) nanoparticle: An adsorption and isotopic separation study

T

⁎

Tao Chen, Qingfeng Wang, Jiafei Lyu, Peng Bai, Xianghai Guo

Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China Key Laboratory of Systems Bioengineering (Ministry of Education), Tianjin University, Tianjin 300072, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Boron Magnetite nanoparticle Adsorption Isotopic separation

Magnetic magnetite nanoparticle (MMN) was studied as an adsorbent for effective boron removal and isotopic separation in aqueous solution for the first time. The adsorption equilibrium was reached rapidly in 1.5 h, and a high capacity of 4.57 mmol/g was achieved at pH = 7 at 45 °C. Quasi second-order kinetic model and Henry isotherm suitably described boron adsorption on magnetite nanoparticles. Furthermore, magnetite nanoparticles showed better affinity for 10B, and a high boron isotopic separation factor (S) of 1.332 was observed at pH = 6 and 15 °C. The adsorption was a spontaneous chemisorption process where the entropy effect is the main driving force instead of enthalpy change. The FTIR and XPS spectra of boron-encapsulated MMN indicated the formation of new FeeOeB bonds. High boron adsorption capacity, excellent boron isotope separation factor, low cost and easy regeneration empower MMN as an adsorbent of great practical value for boron removal and isotope reclamation.

1. Introduction Boron is an important micronutrient for creatures [1], but the amount between excess and deficiency is narrow. Excess intake of boron could damage the central nerves and genital system of people [2]. Besides, high concentration of boron in water could cause harm to vegetation by reducing fruit yield, inducing premature ripening and massive leaf damage [3]. As a result, in some countries the recommended boron concentration range of irrigation water is set between 0.3 and 1 ppm; for potable water, the World Health Organization (WHO) sets the limit at 2.4 mg/L as the acceptable B level, and the limits of some places were even lower [4,5]. However, high boron concentration is detected in seawater [6] at around 5 ppm, and in some groundwater [7] where the concentration can reach up to 119 ppm. This high concentration poses a huge challenge on the water treatment process and seawater desalination. Among reported methods [8–10] for boron removal, adsorption was one of the most observed process because of high adsorption capacity, easy operability, remarkable recycling performance and relatively low cost. Currently, various adsorbents were studied for boron removal including metal-organic frameworks (MOFs) [11,12], ion exchange resins [13], activated carbon [14,15], macromolecular fibers [16] or polymers [17], functional metal [18] or non-metal oxides [19],

membrane materials [20,21], functional magnetic adsorbents [22,23], natural materials [24] and other novel inorganic sorbents [25]. Different magnetic magnetite (Fe3O4) incorporated composites had been fabricated and showed good boron removal performance [23,26]. Although these materials showed positive effect in the boron removal, some drawbacks such as the complicated preparation procedures and restricted boron uptake under low concentrations limited their practical applications in industry. On the other hand, boron has two stable isotopes, boron-10 (10B) and boron-11 (11B), which account for 19.8% and 80.2% in nature, respectively. Specifically, with large thermal-neutron capture crosssection, 10B is widely employed in nuclear and military industry for stabilizing nuclear reactions and improving nuclear fuel efficiency [27]. In addition, 10B plays a critical role in boron neutron capture therapy (BNCT) for cancer treatment [28]. For these important applications, 10B with high abundance is urgently demanded [29]. Therefore, various methods had been explored for efficient boron isotopic separation. In our previous research, adsorption-based chromatographic method proved its potential for boron isotopic separation due to remarkable separation factors and mild separation conditions compared to the traditional chemical exchange distillation. Especially, MOF material MIL-100(Fe) and MIL-101(Cr) showed the highest boron isotopic separation factor so far, which was attributed to the interactions between

⁎ Corresponding author at: Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China. E-mail address: [email protected] (X. Guo).

https://doi.org/10.1016/j.seppur.2019.115930 Received 13 January 2019; Received in revised form 7 August 2019; Accepted 8 August 2019 Available online 09 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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NICOLET 6700) spectra of the materials before and after the adsorption were collected from 4000 to 400 cm−1 in transmittance mode with 8 scans at 4 cm−1 resolution. A porosity and surface area analyzer (GEMINI VII 2390) were used to measure the nitrogen adsorption/ desorption isotherms. Before the test, the specimen was degassed at 100 °C overnight under vacuum. The mean pore size was calculated by the density functional theory (DFT) method and the surface area was figured out through the Brunauer-Emmett-Teller (BET) method. Boron concentration in the aqueous solution was detected by inductively coupled plasma-optical emission spectroscopy (ICP-OES) (PerkinElmer Optima 8000). Inductively coupled plasma mass spectrometry (ICP-MS) (Thermo Electron Corporation, USA) was applied to examine the B isotopic abundance. X-ray photoelectron spectrometry (XPS) were implemented for the exploration of adsorption mechanism by a PHI 1600 ESCA instrument (Al Kα, hν = 1103 eV). C 1s peak at 284.5 eV was used as the reference to calibrate the binding energies. The zero charge (pHzpc) was measured using the Zeta Potential Meter (Brookhaven, Zeta Plus). A Vibrating Sample Magnetometer (VSM, Squid-vsm) from American Quantum was used to analyze nanoparticle magnetic property.

the transition metals and boron in our hypothesis [11]. Inspired by this hypothesis and previous results, magnetic magnetite nanoparticle (MMN) was selected as an adsorbent for boron removal. With tiny particle size and high surface area, MMN could provide huge amounts of active adsorption sites to reinforce adsorbate/adsorbent interactions and selectivity [30,31]. In this contribution, we employed method of co-precipitation of Fe3+ and Fe2+ ions to prepare magnetite nanoparticles as boron adsorbent [32,33]. The boron adsorption properties on MMN were systematically investigated on adsorption kinetics, the influence of temperature and pH, thermodynamics, adsorption mechanism and reusability. We also tested the boron isotopic abundance of prepared boron aqueous solutions before and after adsorption, aiming to evaluate the boron isotopic separation performance on MMN. The results fully supported that MMN could be used as an effective adsorbent for boron removal and boron isotopes reclamation. 2. Experimental section 2.1. Chemicals Reagents in this research were used of analytical-grade without further purification. FeCl2·4H2O (99 wt%), FeCl3·6H2O (99 wt%), ammonia aqueous solution (25 wt%), obtained from Macklin, China. Hydrochloric acid (36 wt%, Jiangtian, China) and sodium hydroxide (99 wt%, Yuanli, China) were applied to adjust solution pH. Glacial acetic acid (99 wt%, Kmart) and deionized (DI) water (Yuanli, China) was employed to prepare desorbent. Boric acid (Yuanli, China) was dissolved in DI water to prepare boron aqueous solutions. All the obtained solutions were stored in polyethene containers instead of glassware in case of the disturbance of boron from glassware.

2.4. Batch adsorption experiments Before the adsorption studies, magnetite (Fe3O4) nanoparticle was activated in a vacuum oven at 60 °C for 10 h. In a typical adsorption experiment, 0.1 g Fe3O4 was added into a 50 mL centrifuge tube, followed by 20 mL boron aqueous solution with specific pH and concentration. The tube was placed in a thermostatic shaker at a specific temperature for boron removal. When the experiment was completed, Fe3O4 was separated by magnetic field and the residual solution was examined for boron concentration and isotopic abundance.

2.2. Preparation of magnetite nanoparticle 2.5. Effect of pH on boron separation

The preparation of Fe3O4 nanoparticle was based on the reported procedure [34,35] with slight modification. FeCl3·6H2O (8.10 g) and FeCl2·4H2O (3.90 g) were added into deionized water (540 mL) under nitrogen atmosphere with stirring. After completely dissolving the two chemicals under vigorous stirring, ammonia aqueous solution (60 mL) was slowly added to adjust the pH to 8 and the color of the mixture gradually turned to black from orange. Subsequently, the black mixture was stirred continuously for 1 h. The obtained particle was separated by a magnet, washed three times with ethanol and then three times with distillated water. Finally, the material was dried in a vacuum oven for 10 h at 40 °C and the yield of Fe3O4 in this study was 98.7% which was slightly lower than the reported result while the FeCl3·6H2O was totally consumed after reaction [36].

The adsorption experiment was carried out with a series of boron solutions at pH = 2–12. The pH was adjusted by 1 mol/L aqueous hydrochloric acid solution or 1 mol/L aqueous sodium hydroxide solution. The activated Fe3O4 was added to these prepared solutions with the dose of 5 g·L−1. The mixtures were placed in a constant temperature shaker at 45 °C for 24 h. The adsorbent was separated by a magnet and the residual solutions were tested for boron concentration and isotopic abundance. Finally, we chose pH 7 with the maximum capacity to continue our following experiments.

2.6. Adsorption kinetics

2.3. Characterization

0.1 g activated Fe3O4 particle was added to boron aqueous solutions (pH = 7) with the initial boron concentrations of 0.1, 0.3, 0.5, and 0.7 mol·L−1, respectively. Then the mixtures were placed in a thermostatic shaker for 24 h at 45 °C. At each time interval, a sample of supernatant was taken for the analysis of boron concentration by ICPOES.

The magnetite nanoparticle was analyzed for phase composition through the X-ray diffraction with anode of Cu (Bruker, λKɑ = 1.5406 Å, Germany). The samples were scanned for 2θ = 10–70° at a scan rate of 3 s per step with a step size of 0.05°. The morphology of the materials was detected on the Scanning Electron Microscope (SEM) (Japan, S-4800, Hitachi) operated at 10 KV. Elementary substance was tested with scanning electron microscopy with energy dispersive X-ray (SEM-EDX) and SEM mapping analysis by choosing random area to find out the distribution of atoms on the surface of adsorbent. Moreover, transmission electron microscope (TEM) analysis was implemented at 200 KeV (Jeol Jem-2100F) to find out the size, shape and surface morphology of magnetic nanoparticle and TEM-EDX was also employed to further find out the display of adsorbate on adsorbent. The weight percent of element of Fe3O4 was also analyzed through X-Ray fluorescence spectrometer (XRF) (Axios, PANalytical). Attenuated total reflection flourier transform infrared spectroscopy (FTIR, Thermo

2.7. Adsorption isotherms The experiments were conducted with a series of boron solutions with specific concentrations (0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.7 mol·L−1) at different temperature (15, 25, 35, 45 °C). The adsorbent was added into these solutions in a dosage of 5 g·L−1 and the mixtures were put into 50 mL plastic tubes in a water bath shaker at 15, 25, 35, 45 °C, respectively. After 24 h, the magnetic particles were separated and the residual solutions were tested for boron concentration with ICP-OES and isotopic abundance with ICP-MS, respectively. 2

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Fig. 1. (A) Comparison of experimental and simulated XRD patterns; (B) SEM image of prepared MMN; (C) Thermogravimetric curve of prepared MMN; (D) Nitrogen adsorption/desorption isotherms.

Fig. 2. (A, B) TEM images of Fe3O4.

Weight loss of prepared nanoparticle was observed at around 200 °C (Fig. 1C), which may be due to the loss of adsorbed water on Fe3O4 surface. The whole weight loss was less than 3% with the increase of temperature and the material was stable up to 800 °C. According to the nitrogen adsorption/desorption isotherms, the Brunauer–Emmett–Teller (BET) surface area of prepared nanoparticles was calculated to be 107 m2·g−1 (Fig. 1D) with NLDFT method. Other porosity properties of the prepared magnetic nanoparticles were summarized in Table S1. The specific surface area was larger than most reported magnetic nanoparticles because of its smaller diameter. The pore volume (Vmi-p) calculated by Dubinin-Radushkevich model was 0.0395 cm3/g for micropores, 0.223 cm3/g for mesopores and 0.015 cm3/g for macropores, respectively. The macropores may be due to the interparticle distance between nanoparticles. The pH of zero point charge (pHzpc) was measured to be 6.7, which was in agreement with reported literature [39,40]. The TEM micrographs (Fig. 2) showed uniform lattice fringes, which confirmed the crystalline structure of nanoparticles [41]. The small quantity of aggregation was mainly resulted from magnetic dipoles forces [42]. The crystallite size in the range of 8–13 nm, which also

2.8. Regeneration and recycling The exhausted Fe3O4 was obtained from the adsorption in 0.7 mol·L−1 of boron at 15 °C. The adsorbent (0.2 g) was regenerated by stirring with 0.01 mol/L hydrochloric acid or 1.5 mol/L acetic acid (20 mL) for 3 h, then washed with water (3 × 20 mL) and activated at 60 °C in a vacuum oven for 10 h. The regenerated particles were examined by XRD and SEM and put back to next cycle.

3. Results and discussion 3.1. Characterization of the prepared Fe3O4 The XRD pattern of prepared Fe3O4 nanoparticles were shown in Fig. 1A compared to the simulated pattern. The diffraction peaks were highly consistent with the simulated pattern (JCPDS No. 75-1609) of face-centered cubic magnetite [37]. According to the peak data of crystal face (3 1 1) with the highest diffraction intensity, the calculated size of prepared Fe3O4 particles by Scherrer Formula [38] was around 11.5 nm, which was in accordance with the SEM results (Fig. 1B). 3

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negative charges instead of B(OH)3 gradually became the main boron species in the solution, which led to an electrostatic repulsion between adsorbent surface and boron species indicated by the sharp drop of the adsorption capacity at pH range of 7–12. As a result, the electrostatic forces between boron species and surface groups of adsorbents had significant effects on the sorption.

matched the calculated size by XRD pattern. As reported by S. Japip, Y. Xiao and T.S. Chung [43] that particle of small size could be obtained with lower concentration of metal salt during synthesis. So, the smaller particle size in this research [42,44–47] was probably because of the lower concentration of employed Fe salts as well as reduced reaction temperature. The weight percent of the adsorbent was reported as oxides on an ash basis by conversion. The results of Table S2 confirmed that Fe (99.225% Fe2O3) was the main metal constituent after washing of magnetite nanoparticle. One of the advantages of applying Fe3O4 nanoparticles in practice was that they could be easily separated by external magnetic field. The magnetic properties of the prepared adsorbent were shown as (M-H) hysteresis plot in Fig. S3. The saturation magnetization value was 76.54 emu/g versus applied magnetic field (H) plots at 300 K and the M value was roughly equal to previous reports [39,44].

3.3. Adsorption kinetics The adsorption kinetics were obtained by soaking 0.1 g activated Fe3O4 in boron solutions with different initial concentrations at 45 °C (Fig. 4). The adsorbing amount grew fast in the first one hour and reached adsorptive equilibrium in 1.5 h (Fig. 4A). The equilibrium capacity (Qe) increased from 7.67 mg·g−1 (0.71 mmol·g−1) to 49.36 mg·g−1 (4.57 mmol·g−1) with initial boron concentration from 0.1 mol·L−1 to 0.7 mol·L−1. Quasi second-order and intraparticle diffusion kinetic models were applied to explain the kinetic data (Fig. 4B and C). The pseudo secondorder model can be expressed as following:

3.2. Effect of pH on boron adsorption

t t 1 = + Qt Qe D2 Qe2

In aqueous solution, boron exists in different forms at different pHs [10]. In general, there are two main boron complexes, borate ion [B (OH)4−] and non-ionized boric acid [B(OH)3] [48]. The increase of pH causes the decrease of B(OH)3 concentration and the increase of [B (OH)4−] concentration. In addition, the pH can determine the charge on the adsorbent surface [39]. As a result, pH would exert great influence on boron adsorption and isotopic separation performance. Fig. 3 showed the results of boron uptake on the magnetic nanoparticles at pH range of 2 to 12. Specifically, the adsorption capacity increased from pH 2 to pH 7 and reached a maximum amount (4.57 mmol/g) at pH = 7. Then, it quickly decreased from pH = 8 and the decrease slowed down from pH 10 to pH 12. When protonation and deprotonation occurred to the surface of magnetite particles at pH below or above the pHzpc respectively, the surface of Fe3O4 would be covered with (FeOH)2+ and (FeOH)+, or [Fe (OH)3]− and [Fe(OH)4]− ionic groups in aqueous solution, which were called as hydrated MMN ions (HMMN+ and HMMN−) [32,49,50]. The obtained pHzpc(6.7) was close to the pH with boron maximum capacity, where the concentrations of HMMN+ and HMMN− groups on adsorbent surface were equal [51]. At pH < 7, MMN was in the form of HMMN+ with positive surface charge, which may prefer to adsorb B (OH)4− than H3BO3 due to the electrostatic attraction. At the meantime, the amount of B(OH)4− will gradually increase with the pH rising, so the adsorption capacity of boron would increase from pH = 2 to pH = 7. By further increasing the pH from 7 to 12, HMMN− groups on surface of magnetite were formed, at the same time the B(OH)4− with

(1) −1

−1

where D2 (g·mmol ·h ) was the rate constant of Quasi second-order model. Qt and Qe (mg·g−1) were the adsorption capacity at time t (h) and at equilibrium. The Qe and D2 were calculated according to the slopes and intercepts of the curves in Fig. 4B. Besides, the data of the kinetic experiment were also correlated with the intraparticle diffusion model to explore the influence of diffusion process during adsorption. The model can be described as following:

Qt = Di ·t 0.5 + K

(2)

where Qt (mg·g−1) was the boron adsorption capacity at time t (h), Di (mg·g−1·h−0.5) was the diffusion rate constant and K was the intercept of the curves shown in the Fig. 4(C). Larger K implied more impacts from boundary layer on the adsorption process. Relevant parameters of two kinetic models (Table 1) showed that boron adsorption on Fe3O4 can be well fitted by the quasi second-order model, which indicated that the chemisorption process could be the rate-controlling step [12]. So electron transfer between adsorbate and Fe3O4 molecules [52] might occur and the adsorption rate largely depended on the number of accessible magnetite surface adsorptive sites [53,54]. In intraparticle diffusion model, the values of K decreased with the increasing initial boron concentration, indicating that the stronger driving force improved the intraparticle diffusion and less impact from the bounding layer on the boron adsorption at high concentrations [52] (Table 1). In addition, Boyd kinetic was implemented to study the actual controlling step of boron adsorption on Fe3O4. This model can be expressed by Eqs. (3) and (4):

6 F = 1 − ⎛ 2 ⎞·exp(−Bt ) ⎝π ⎠

F=

Qt Qe

(3)

(4) −1

where Qt and Qe (mg·g ) were the boron adsorption amount at time t (h) and at equilibrium, respectively. F represented the ratio of absorption capacity at t to that at equilibrium. Bt can be figured out through the Reichenberg Eq. (5):

Bt = −ln(1 − F ) − 0.4977

(5)

The linear correlation of Bt versus t can be applied to determine whether the particle diffusion or film diffusion controlled the process of boron adsorption on Fe3O4. If fine linearity was observed with the line crossing the zero point, the adsorption process was controlled by the particle diffusion. On the contrary, the process was controlled by film

Fig. 3. Boron adsorption capacity at different pHs. 4

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Fig. 4. (A) Time-dependent boron adsorption on Fe3O4 (dosages: 5 g·L−1, temperature: 45 °C); (B) Quasi second-order kinetics of boron adsorption on Fe3O4; (C) Intraparticle diffusion kinetics of boron adsorption on Fe3O4; (D) Boyd plot for boron adsorption on Fe3O4.

diffusion instead of particle diffusion [55]. In this research, the curve of Bt versus t was parabolic and did not go through the zero point (Fig. 4D), which indicated that the external film diffusion should mainly control the adsorption process.

surface [60], which is in accordance with the analyses from adsorption kinetics.

3.4. Adsorption isotherms

The thermodynamic parameters including enthalpy change(ΔH), entropy change(ΔS) and Gibbs free energy change(ΔG) were obtained by the following equations:

3.5. Adsorption thermodynamics

Three models including Langmuir, Henry and Freundlich were applied to fit the boron adsorption isotherms on magnetic nanoparticles (Fig. 5A–D). Relevant parameters of these models and corresponding plots were concluded in Table 3. The adsorption capacities were found to increase with the rising temperature, suggesting the boron adsorption on Fe3O4 was an endothermal process. The results showed that the experimental data could be fitted well to all three models with high correlation coefficient (R2). Specially, Freundlich model will be identical to the Langmuir isotherm mathematically [56] when the index of heterogeneity (n) near to 1 (Table 2). In some reports, chemisorption was determined as the main process when the isotherms fitted both Langmuir and Freundlich model well [57–59]. The theoretical maximum capacity Qm (211 mmol/g) at 45 °C calculated by Langmuir model was far larger than those of previous researches, which might be unachievable due to the solubility limit of boric acid. Since the data were well fitted to the Langmuir model, the boron adsorption on magnetite should be a chemisorption process on the homogenous

ln Kp =

ΔS ΔH − R RT

(6)

ΔG = −RT ·ln Kp

(7)

where R was a constant equal to 8.314 J·mol−1·K−1; Kp was the distribution factor that can be figured out from the Henry model (Table 2); and T was the Kelvin temperature (K). The values of ΔH, ΔS and ΔG, determined by Van’t Hoff plot (Fig. 6), were summarized in Table 3. The positive ΔH (3.33 kJ·mol−1) further indicated the boron adsorption on the Fe3O4 was an endothermic process. The entropy change ΔS (27.1 J·mol−1·K−1) suggested increased randomness at the interface of adsorbent-adsorbate due to released water molecules. As a result, the determined negative ΔG at studied temperatures confirmed the boron adsorption on the Fe3O4 was the spontaneous process, and the entropy effect should be the primary driving force for the B adsorbing on

Table 1 Kinetic parameters of different models for boron adsorption on Fe3O4. C0 mol·L−1

0.1 0.3 0.5 0.7

Qe (exp.) mmol·g−1

0.71 1.99 3.48 4.57

Quasi second-order model

Intraparticle diffusion model

Qe (cal.) mmol·g−1

D2 g·mmol−1·h−1

R

K

Di mg·g−1·h−0.5

R2

0.72 2.06 3.64 4.81

13.57 3.08 1.57 1.01

0.999 0.999 0.998 0.997

3.1 4.17 2.17 1.86

4.27 16.32 34.16 50.71

0.772 0.660 0.838 0.856

5

2

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Fig. 5. (A) Isotherms of boron adsorption on Fe3O4; (B) Fitting curves of Langmuir model; (C) Fitting curves of Henry model; (D) Fitting curves of Freundlich model. Table 2 Parameters of three isotherm models for boron adsorption on Fe3O4. T °C

15 25 35 45

Langmuir model

Freundlich model

Qm mol·g−1

KA L·mol−1

R2

0.094 0.158 0.184 0.211

0.069 0.041 0.037 0.032

0.997 0.984 0.998 0.999

n

1.09 1.15 1.29 1.11

KF

0.093 0.086 0.089 0.108

Table 3 Calculated thermodynamic parameters for boron adsorption on Fe3O4.

Henry model R2

0.959 0.986 0.998 0.997

Kp mL·g−1

R2

6.47 6.79 7.24 7.32

0.999 0.999 0.999 0.999

T °C

ΔG kJ·mol−1

ΔS J·mol−1·K−1

ΔH kJ·mol−1

15 25 35 45

−4.47 −4.75 −5.07 −5.27

27.1

3.33

boron concentration and isotope abundance before and after boron adsorption. According to precious report [61], the equation was shown as following:

S⎛ ⎝ ⎜

10B 11B

⎞= ( ( ⎠ ⎟

10B/ 11B)sorbent 10B/ 11B)solution

=

C0 ∂0 (1 + ∂1) − C1 ∂1 (1 + ∂0) C0 ∂1 (1 + ∂1) − C1 ∂1 (1 + ∂0)

(8)

where C0 (mol·L−1) was the initial boron concentration, C1 (mol·L−1) was the boron concentration after adsorption, ∂0 represented initial 10 B/11B abundance, ∂1represented 10B/11B abundance of the solution after adsorption. Change of the temperature and the pH of solution would affect the adsorption capacity as well as the separation factor. As a result, four temperatures (15 °C, 25 °C, 35 °C, 45 °C) at pH = 6 and pHs (2–12) at 15 °C with an initial boron concentration of 0.7 mol·L−1 were investigated upon isotopic separation factor in this study. Unlike some other boron isotopic separation materials [10,11,61], MMN showed preference to 10B instead of 11B. Different from the adsorption capacity tendency upon the temperature, the isotopic separation factor gradually decreased upon rising temperature (Fig. 7A). The lower separation factor at higher temperature can be attributed to the non-selective affinity between boron isotopes and the Fe3O4. As shown in Fig. 7B, the highest separation factor (1.332) was observed at pH = 6 where the adsorption capacity (2.46 mmol/g) was less than the maximum capacity (4.57 mmol/g) observed at pH = 7 (Fig. 3). The second highest separation factor (1.279) was achieved at pH = 7 where the surface of magnetic nanoparticles was near the isopotential point pHzpc (6.7). In practice, the operating point should be within pH = 6–7 in consideration of the balance between adsorptive capacity and isotopic

Fig. 6. Van’t Hoff plot.

magnetic nanoparticles.

3.6. Effect of temperature and pH on boron isotopic separation factor The boron isotopes separation behavior of sorbents can be determined by the separation factor which can be calculated based on the 6

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Fig. 7. Effect of temperature (A) and pH (B) on boron isotopic separation on Fe3O4.

separation factor. Compared to the sorbents in previous reports, the prepared magnetic nanoparticles in this study showed relatively high Qm (4.57 mmol/g) and S (1.332) (Tables S3 and S4).

Table 4 Elemental composition of Fe3O4 before and after boron adsorption. Elements

3.7. Boron adsorption mechanism on Fe3O4

Iron Oxygen Sodium Carbon Boron

To illustrate the possible adsorptive mechanism, TEM elemental mapping, FT-IR and XPS were employed to characterize the changes of MMN before and after adsorbing boron. TEM (Fig. 8) and SEM (Figs. S2 and S3) elemental mapping were conducted to show the distributions of iron, oxygen, and boron in the nanoparticles after adsorption. The details of EDX analyses was summarized in Table 4. Intense iron and oxygen peaks can be found on prepared Fe3O4 while tiny amounts of boron in prepared Fe3O4 as background interference may due to the B existing in the air. However, more obvious boron peak can be found after the adsorption process (Fig. S2B) compared with Fig. S2A. The weight percentage of B after adsorption (4.83%) matched the adsorption capacity (4.57 mmol/g, 4.93%) of Fe3O4 applied in 0.7 M boron aqueous solution at 45 °C. The same results can also be found in SEM mapping results in Fig. S3. According to reported research [62], four different species of boron compounds including H3BO3, B(OH)4−, B3O3(OH)4− and B3O3(OH)52− exist in aqueous solution of boric acid

Fe3O4 before boron adsorption

Fe3O4 after boron adsorption

wt.%

at.%

wt.%

at.%

58.59 32.58 2.415 6.39 0.025

28.24 54.80 2.668 14.24 0.052

55.92 33.54 2.75 5.81 1.98

25.86 54.05 2.86 12.49 4.83

at. %: Atomic percentage.

where the amount of H3BO3 and B(OH)4− was far larger than the other two species [48]. As a result, the main forms of B, H3BO3 and B(OH)4−, were studied as the adsorbate absorbing in MMN in this study. The FT-IR spectra (Fig. 9) of nanoparticle showed vibration and stretching of functional groups on magnetic nanoparticles before and after absorbing boron. The stretching band of OeH on the surface of Fe3O4 [63] at 3429 cm−1 shifted to 3398 cm−1 after adsorption, probably caused by the formed hydrogen bonds during adsorption between boric acid and MMN [64]. BeO asymmetric stretching band at 1404 cm−1 was observed on MMN after adsorption [65], which

Fig. 8. TEM elemental mapping images of boron-loaded adsorbents. 7

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adsorption of boron (Fig. 10A). The binding energies at Fe 2p3/2, Fe 2p1/2 decreased after boron was adsorbed on Fe3O4 (Fig. 10B), which suggested some additional electron density may be provided by oxygen atom to Fe atom, leading to a possible formation of FeeOeB bond [70]. The binding energy of O 1s (Fig. 10C) did increase after adsorption, which supported oxygen and Fe atom would compensate with each other. The decreasing binding energy of B 1s from 193.4 eV to 191.8 eV after adsorption (Fig. 10D) may be also due to the forming of new bond during adsorption [71]. XPS spectra of MMN before and after adsorption were deconvoluted Fig. 11 where O1s spectrum were fitted into two peaks before adsorption and three peaks or four peaks after adsorbing boron acid by means of least squares curve fitting. The O1s spectrum before adsorption was deconvoluted into two peaks (Fig. 11A) at the binding energies of approximately 529.97 eV and 531.27 eV corresponding to the FeeOeFe and FeeOeH bonds, respectively [72,73]. After boron adsorption, a new peak showed up at 533.51 eV representing the BeOeH bond. By further introducing the FeeOeB bond with binding energy of 531.5 eV into the deconvolution, the squared deviation value (χ2) largely decreased from 24.36 to 0.522, indicating the probably forming of the FeeOeB bonds during boron adsorbing in MMN.

Fig. 9. FT-IR spectra of MMN before and after adsorption.

indicated that the boron was successfully adsorbed on MMN. The FeeO vibration peak shift from 586 cm−1 to 582 cm−1 after adsorption [66,67], might be caused by some new atom (B) attached to this bond between adsorbate and FeeO [66]. Peak at 1630 cm−1 indicated that there might be some binding water molecules on the Fe3O4 surface after boron adsorption [68]. The wavenumber changes of OeH, BeO and FeeO bonds after adsorption indicated the adsorption of boron on MMN may happen through the form of new bond of FeeOeB [69]. To further study the mechanism of boron adsorption on magnetic nanoparticles, XPS was implemented to analyze the boron adsorbed on Fe3O4 and the interaction sites during adsorption. The presence of B 1s on the surface of MMN after sorption clearly proved the successful

3.8. Regeneration Three adsorption-desorption cycles were conducted to study the adsorbent reusability. 0.01 M hydrochloric acid and 1.5 M acetic acid were employed to desorb the boron from magnetic nanoparticles, respectively. The results showed that the adsorptive capacity decreased slightly and the isotopic separation factor kept stable after three cycles (Fig. 12A and B). Although both acid solutions could effectively regenerate the adsorbent, the 0.01 M HCl solution showed a little bit higher recovery than that of acetic acid. The XRD pattern (Fig. S4C) and SEM image (Fig. S5B) after three cycles proved no significant change of

Fig. 10. High-resolution XPS spectra of (A) MMN; (B) Fe 2p; (C) O 1s before and after adsorption; (D) B 1s of boron acid, MMN before and after adsorption. 8

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Fig. 11. (A) Deconvolution of XPS spectra of O1s in MMN before adsorption; (B) After adsorption fitting into three peaks and (C) four peaks.

Fig. 12. (A) Recycling of Fe3O4 on boron adsorption (15 °C, pH = 7, C0 = 0.7 M); (B) Separation factor of Fe3O4 on boron isotope separation in three cycles.

crystalline structure and morphology compared with the raw material (Figs. S4B and S5A). The calculated particle size by the XRD pattern (Fig. S4C) of recycled Fe3O4 was about 10.7 nm, indicating no obvious change with the material after adsorption. All of these results demonstrated the stability of prepared nanoparticles during the process of boron removal.

(No. 16JCYBJC20300).

4. Conclusions

References

Magnetic Fe3O4 nanoparticles were successfully synthesized through a facile co-precipitated method with high specific surface area and super magnetism. Boron adsorption on magnetite was investigated on kinetics, isotherms, effect of pH and temperature, mechanism and reusability. The results showed that the boron adsorption on Fe3O4 was an endothermic process dominated by external film diffusion and entropy change. The magnetic nanoparticles exhibited high adsorption capacity and isotopic separation factor at mild operating conditions. In addition, with low cost, facile synthesis, satisfactory mechanical strength, easy regeneration and ability to be separated by external magnetic field, the magnetic Fe3O4 can be a promising agent for boron removal and isotopic separation in industrial application.

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Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.115930.

Note The authors declare no competing financial interest. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments We gratefully acknowledge the funding for this work provided by Independent Innovation Foundation of Tianjin University of China (No. 2017XZY-0052), and Natural Science Foundation of Tianjin of China 9

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